File:Looking down from The Eiffel Tower, Paris 8 April 2007.jpg|thumb|Shadows of visitors to the Eiffel Tower, viewed from the first platform
File:Park grid.JPG|thumb|Park fence shadow is distorted by an uneven snow surface
A '''shadow''' is a dark area where light
from a light source is blocked by an Opacity (optics)|opaque
object. It occupies all of the three-dimensional volume behind an object with light in front of it. The cross section (geometry)|cross section
of a shadow is a two-dimension
, or a reverse Projection (linear algebra)|projection
of the object blocking the light.